1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting and receiving a signal in a communication system, and in particular, to an apparatus and method for transmitting and receiving a signal using structured Low Density Parity Check (LDPC) codes in a communication system.
2. Description of the Related Art
The next generation communication system has evolved into a packet service communication system. The packet service communication system, a system for transmitting burst packet data to a plurality of mobile stations (MSs), has been designed for high-capacity data transmission.
Various data transmission schemes, such as a Hybrid Automatic Repeat reQuest (HARQ) scheme and an Adaptive Modulation and Coding (AMC) scheme, have been proposed for the next generation communication system in order to increase data throughput. To support the HARQ scheme and the AMC scheme, the communication system must support various coding rates.
Commonly, the next generation communication system uses LDPC codes together with turbo codes. It is well known that the LDPC codes show a high performance gain when they are used for high-speed data transmission, and the LDPC codes effectively correct errors caused by noises generated in a transmission channel, thereby contributing to an increase in reliability of data transmission.
FIG. 1 is a block diagram illustrating a conventional communication system using LDPC codes. Referring to FIG. 1, the communication system includes a signal transmission apparatus 100 and a signal reception apparatus 150. The signal transmission apparatus 100 includes an encoder 111, a modulator 113, and a radio frequency (RF) processor 115, and the signal reception apparatus 150 includes an RF processor 151, a demodulator 153, and a decoder 155. If there is an information vector s that the signal transmission apparatus 100 desires to transmit, the information vector s is provided to the encoder 111. The encoder 111 encodes the information vector s into a codeword vector c, i.e., an LDPC codeword, using a predetermined coding scheme, and outputs the generated codeword vector c to the modulator 113. Herein, the predetermined coding scheme refers to an LDPC coding scheme. The modulator 113 modulates the codeword vector c into a modulation vector m using a predetermined modulation scheme, and outputs the modulation vector m to the RF processor 115. The RF processor 115 RF-processes the modulation vector m output from the modulator 113, and transmits the RF-processed signal to the signal reception apparatus 150 via an antenna ANT.
The signal transmitted by the signal transmission apparatus 100 is received at the signal reception apparatus 150 via its antenna ANT, and the received signal is provided to the RF processor 151. The RF processor 151 RF-processes the received signal, and outputs the RF-processed vector r to the demodulator 153. The demodulator 153 demodulates the vector r output from the RF processor 151 using a demodulation scheme corresponding to the modulation scheme used in the modulator 113 of the signal transmission apparatus 100, and outputs the demodulated vector x to the decoder 155. The decoder 155 decodes the vector x output from the demodulator 153 using a decoding scheme corresponding to the coding scheme used in the encoder 111 of the signal transmission apparatus 100, and finally outputs the decoded signal ŝ as a restored information vector.
FIG. 2 is a diagram illustrating a parity check matrix of a conventional LDPC code. Referring to FIG. 2, a parity check matrix of the LDPC code is formed such that the full parity check matrix is divided into a plurality of blocks, and permutation matrixes are mapped to the individual blocks. It will be assumed herein that the permutation matrixes each have a size of Ns×Ns.
As illustrated in FIG. 2, the parity check matrix of the structured LDPC code is divided into m×n blocks, and a permutation matrix is mapped to each of the m×n blocks. Pmn represents a permutation matrix located in a point where an mth block row and an nth block column among the plurality of blocks in the parity check matrix cross each other. The permutation matrix mapped to each of the blocks will be referred to as a “block matrix.” In a parity check matrix in which identity matrixes are selected for all the block matrixes, once a point of a non-zero element in a first row of each block is determined, points of the remaining (Ns−1) non-zero elements are determined. Therefore, the memory capacity required for storing the full information on the parity check matrix is reduced to 1/Ns as compared with the required memory capacity where points of the non-zero elements are irregularly selected.
The communication system uses various error control schemes for improvement of system reliability, and the Hybrid Automatic Repeat reQuest (HARQ) scheme is a scheme obtained by combining advantages of a Forward Error Correction (FEC) scheme and an Automatic Repeat reQuest (ARQ) scheme among the error control schemes. The HARQ scheme, i.e., a scheme for reducing the number of retransmissions by correcting a frequently generated error pattern using the FEC scheme, is classified into three types: Type-I, Type-II, and Type-III. A description will now be made of the three types of the HARQ scheme.
FIG. 3 is a diagram schematically illustrating a Type-I HARQ-based signal transmission and reception operation in a conventional communication system. However, before a description of FIG. 3 is given, it should be noted that the Type-I HARQ-based signal transmission and reception operation can be performed using either one code having both an error correction function and an error detection function, or two different codes, one of which has the error correction function and the other of which has the error detection function. Further, it is assumed in FIG. 3 that the Type-I HARQ-based signal transmission and reception operation is performed using the two different codes, one of which has the error correction function and the other of which has the error detection function.
Referring to FIG. 3, in the Type-I HARQ scheme, a signal transmission apparatus transmits codeword vectors in the same format at both initial transmission and retransmissions. That is, at initial transmission, the signal transmission apparatus encodes a k-bit information vector into a codeword vector (k′,k) for error detection through a first encoder using a predetermined coding scheme, for example, a turbo coding scheme supporting a predetermined fixed coding rate, encodes again the codeword vector (k′,k) into a codeword vector (n,k′) for error correction through a second encoder using a predetermined coding scheme, for example, a turbo coding scheme supporting a predetermined fixed coding rate, and then transmits the codeword vector (n,k′) as a final codeword vector.
Thereafter, at retransmission caused by an error occurred in the initially transmitted codeword vector, the signal transmission apparatus transmits the codeword vector (n,k′) that was transmitted at initial transmission. Herein, a coding rate of the codeword vector (n,k′) is assumed to be R0, and because the codeword vector (n,k′) was encoded by the turbo coding scheme, it is comprised of an information part S0 mapped to the information vector and a parity part (P00, P01) mapped to a parity vector. As a result, in the Type-I HARQ scheme, the same coding rate R0 is used for initial transmission and retransmissions, and thus the same codeword vector (n,k′) is transmitted.
Upon receiving a codeword vector initially transmitted by the signal transmission apparatus, a signal reception apparatus decodes the received codeword vector through a first decoder using a decoding scheme corresponding to the coding scheme used in the signal transmission apparatus in order to correct an error in the received codeword vector. The decoding operation of the first decoder corresponds to an operation of encoding the codeword vector (n,k′) in the second encoder of the signal transmission apparatus, and the received codeword vector, when it is correctly error-corrected, is restored to the codeword vector (k′,k).
After error-correcting the received codeword vector, the signal reception apparatus decodes the error-corrected received codeword vector through a second decoder using a decoding scheme corresponding to the coding scheme used in the signal transmission apparatus, thereby detecting an error in the error-corrected received codeword vector. The decoding operation of the second decoder corresponds to an operation of encoding the codeword vector (k′,k) in the first encoder of the signal transmission apparatus. Upon detecting an error in the error-corrected received codeword vector, the signal reception apparatus transmits negative acknowledgement (NAK) information indicating abnormal receipt of the initially transmitted codeword vector to the signal transmission apparatus in order to request retransmission of the corresponding codeword vector. The signal reception apparatus temporarily buffers the error-detected codeword vector in its buffer, preparing to combine the error-detected codeword vector with a retransmitted codeword vector. However, upon detecting no error in the error-corrected received codeword vector, the signal reception apparatus transmits acknowledgement (ACK) information indicating normal receipt of the initially transmitted codeword vector to the signal transmission apparatus.
If the signal transmission apparatus retransmits the codeword vector (n,k′) that it transmitted at initial transmission, in response to the retransmission request for the codeword vector from the signal reception apparatus, the signal reception apparatus receives the codeword vector retransmitted by the signal transmission apparatus, error-corrects the received codeword vector through the first decoder, combines the error-corrected received codeword vector with the error-detected initially-transmitted codeword vector buffered therein, and then detects a possible error in the combined received codeword vector through the second decoder. Accordingly, the signal transmission apparatus and the signal reception apparatus repeatedly perform the Type-I HARQ-based signal transmission and reception operation until the k-bit information vector is normally restored within a predetermined number of retransmissions, or within a predetermined time.
As described above, because the signal transmission apparatus transmits the same codeword vector both at initial transmission and at subsequent retransmissions, the Type-I HARQ-based signal transmission and reception operation abruptly decreases system throughput in a poor channel condition.
FIG. 4 is a diagram schematically illustrating a Type-II HARQ-based signal transmission and reception operation in a conventional communication system. However, before a description of FIG. 4 is given, it should be noted that the Type-II HARQ-based signal transmission and reception operation can be performed using either one code having both an error correction function and an error detection function, or two different codes, one of which has the error correction function and the other of which has the error detection function. Further, it is assumed in FIG. 4 that the Type-II HARQ-based signal transmission and reception operation is performed using the two different codes, one of which has the error correction function and the other of which has the error detection function.
Referring to FIG. 4, in the Type-II HARQ scheme, a signal transmission apparatus transmits codeword vectors in different formats at initial transmission and retransmissions. That is, at initial transmission, the signal transmission apparatus encodes a k-bit information vector into a codeword vector (k′,k) for error detection through a first encoder using a predetermined coding scheme, for example, a turbo coding scheme supporting a predetermined fixed coding rate, encodes again the codeword vector (k′,k) into a codeword vector (n,k′) for error correction through a second encoder using a predetermined coding scheme, for example, a turbo coding scheme supporting a predetermined fixed coding rate, and then transmits the codeword vector (n,k′) as a final codeword vector. If it is assumed that coding rates supportable by the codeword vector (n,k′) are R0, R1, . . . , RL (where R0>R1> . . . >RL), the signal transmission apparatus transmits a codeword vector (n,k′)(R0) at initial transmission, where (n,k′)(R0) denotes a codeword vector (n,k′) at a coding rate R0. Further, the codeword vector (n,k′)(R0), because it was encoded by the turbo coding scheme, is comprised of an information part S0 mapped to the information vector and a parity part (P00, P01) mapped to a parity vector.
Thereafter, at a retransmission caused by an error occurred in the initially transmitted codeword vector, the signal transmission apparatus transmits, to the signal reception apparatus, only the additional parity vector P1 prepared for the codeword vector (n,k′)(R0) for a codeword vector (n,k′)(R1) being different from the codeword vector (n,k′)(R0) that was transmitted at initial transmission. The codeword vector (n,k′)(R1) includes an information part S0 mapped to the information vector and a parity part (P00, P01) and a parity part P1 mapped to a parity vector. As a result, the signal transmission apparatus transmits only the parity part P1 to the signal reception apparatus at retransmission.
Upon receiving a codeword vector initially transmitted by the signal transmission apparatus, a signal reception apparatus decodes the received codeword vector through a first decoder using a decoding scheme corresponding to the coding scheme used in the signal transmission apparatus in order to correct an error in the received codeword vector. The decoding operation of the first decoder corresponds to an operation of encoding the codeword vector (n,k′) in the second encoder of the signal transmission apparatus, and the received codeword vector, when it is correctly error-corrected, is restored to the codeword vector (k′,k).
After error-correcting the received codeword vector, the signal reception apparatus decodes the error-corrected received codeword vector through a second decoder using a decoding scheme corresponding to the coding scheme used in the signal transmission apparatus, thereby detecting an error in the error-corrected received codeword vector. The decoding operation of the second decoder corresponds to an operation of encoding the codeword vector (k′,k) in the first encoder of the signal transmission apparatus.
Upon detecting an error in the error-corrected received codeword vector, the signal reception apparatus transmits NAK information indicating abnormal receipt of the initially transmitted codeword vector to the signal transmission apparatus in order to request retransmission of the corresponding codeword vector. The signal reception apparatus temporarily buffers the error-detected codeword vector in its buffer, preparing to combine the error-detected codeword vector with a retransmitted codeword vector. However, upon detecting no error in the error-corrected received codeword vector, the signal reception apparatus transmits ACK information indicating normal receipt of the initially transmitted codeword vector to the signal transmission apparatus.
If the signal transmission apparatus retransmits only the parity vector P1 added to the codeword vector (n,k′)(R0) that it transmitted at initial transmission, in response to the retransmission request for the codeword vector from the signal reception apparatus, the signal reception apparatus receives the codeword vector retransmitted by the signal transmission apparatus, error-corrects the received codeword vector through the first decoder, combines the error-corrected received codeword vector with the error-detected initially-transmitted codeword vector buffered therein, and then detects a possible error in the combined received codeword vector through the second decoder. Accordingly, the signal transmission apparatus and the signal reception apparatus repeatedly perform the Type-II HARQ-based signal transmission and reception operation until the k-bit information vector is normally restored within a predetermined number of retransmissions, or within a predetermined time.
As described above, in the Type-II HARQ-based signal transmission and reception operation, the signal transmission apparatus transmits only the additional parity vector for the previously transmitted codeword vector at retransmission. Therefore, if there is a fatal error in the codeword vector initially transmitted by the signal transmission apparatus, the signal reception apparatus may occasionally fail to correctly restore the information vector. Therefore, the signal transmission apparatus retransmits the initially transmitted codeword vector periodically, for example, at every predetermined number, L, of retransmissions, thereby enabling normal restoration of the information vector.
However, because the Type-II HARQ-based signal transmission and reception operation must generate a codeword vector (n,k′) supporting a variable coding rate at retransmission, the signal transmission apparatus should include additional encoders to generate the codeword vector (n,k′) and the signal reception apparatus should also include additional decoders to decode the codeword vector (n,k′). In addition, at retransmission, the signal transmission apparatus transmits only the parity vector added to the initially transmitted codeword vector, instead of transmitting the intact codeword vector (n,k′) supporting the variable coding rate, so it must include puncturers for puncturing the remaining parts except for the additional parity vector. As a result, the Type-II HARQ-based signal transmission and reception operation causes an undesirable increase in hardware complexity.
FIG. 5 is a diagram schematically illustrating a Type-III HARQ-based signal transmission and reception operation in a conventional communication system. However, before a description of FIG. 5 is given, it should be noted that the Type-III HARQ-based signal transmission and reception operation can be performed using either one code having both an error correction function and an error detection function, or two different codes, one of which has the error correction function and the other of which has the error detection function. Further, it is assumed in FIG. 5 that the Type-III HARQ-based signal transmission and reception operation is performed using the two different codes, one of which has the error correction function and the other of which has the error detection function.
Referring to FIG. 5, in the Type-III HARQ scheme, a signal transmission apparatus transmits codeword vectors in different formats at initial transmission and retransmissions. That is, at an initial transmission, the signal transmission apparatus encodes a k-bit information vector into a codeword vector (k′,k) for error detection through a first encoder using a predetermined coding scheme, for example, a turbo coding scheme supporting a predetermined fixed coding rate, encodes again the codeword vector (k′,k) into a codeword vector (n,k′) for error correction through a second encoder using a predetermined coding scheme, for example, a turbo coding scheme supporting a predetermined fixed coding rate, and then transmits the codeword vector (n,k′) as a final codeword vector.
If it is assumed that coding rates supportable by the codeword vector (n,k′) are R0, R1, . . . , RL (where R0>R1> . . . >RL), the signal transmission apparatus transmits a codeword vector (n,k′)(R0) at initial transmission, where (n,k′)(R0) denotes a codeword vector (n,k′) at a coding rate R0. The codeword vector (n,k′)(R0), because it was encoded by the turbo coding scheme, includes an information part S0 mapped to the information vector and a parity part (P00, P01) mapped to a parity vector.
Thereafter, at retransmission caused by an error occurred in the initially transmitted codeword vector, the signal transmission apparatus transmits, to the signal reception apparatus, a codeword vector being different from the codeword vector (n,k′)(R0) that was transmitted at initial transmission, i.e., an information part S0 and an additional parity vector P1 prepared for the codeword vector (n,k′)R0 for a codeword vector (n,k′)(R1). The codeword vector (n,k′)(R1) includes the information part S0 mapped to the information vector and the parity part (P00, P01) and the parity part P1 mapped to a parity vector. As a result, the signal transmission apparatus transmits the information part S0 and the parity part P1 to the signal reception apparatus at retransmission.
Upon receiving a codeword vector initially transmitted by the signal transmission apparatus, a signal reception apparatus decodes the received codeword vector through a first decoder using a decoding scheme corresponding to the coding scheme used in the signal transmission apparatus, thereby to correct an error in the received codeword vector. The decoding operation of the first decoder corresponds to an operation of encoding the codeword vector (n,k′) in the second encoder of the signal transmission apparatus, and the received codeword vector, when it is correctly error-corrected, is restored to the codeword vector (k′,k).
After error-correcting the received codeword vector, the signal reception apparatus decodes the error-corrected received codeword vector through a second decoder using a decoding scheme corresponding to the coding scheme used in the signal transmission apparatus, thereby detecting an error in the error-corrected received codeword vector. The decoding operation of the second decoder corresponds to an operation of encoding the codeword vector (k′,k) in the first encoder of the signal transmission apparatus.
Upon detecting an error in the error-corrected received codeword vector, the signal reception apparatus transmits NAK information indicating abnormal receipt of the initially transmitted codeword vector to the signal transmission apparatus in order to request retransmission of the corresponding codeword vector. The signal reception apparatus temporarily buffers the error-detected codeword vector in its buffer, preparing to combine the error-detected codeword vector with a retransmitted codeword vector. However, upon detecting no error in the error-corrected received codeword vector, the signal reception apparatus transmits ACK information indicating normal receipt of the initially transmitted codeword vector to the signal transmission apparatus.
If the signal transmission apparatus retransmits the information part S0 and the parity vector P1 other than the codeword vector (n,k′)(R0) that it transmitted at initial transmission, in response to the retransmission request for the codeword vector from the signal reception apparatus, the signal reception apparatus receives the codeword vector retransmitted by the signal transmission apparatus, error-corrects the received codeword vector through the first decoder, combines the error-corrected received codeword vector with the error-detected initially-transmitted codeword vector buffered therein, and then detects a possible error in the combined received codeword vector through the second decoder. Accordingly, the signal transmission apparatus and the signal reception apparatus repeatedly perform the Type-III HARQ-based signal transmission and reception operation until the k-bit information vector is normally restored within a predetermined number of retransmissions, or within a predetermined time.
As described above, in the Type-III HARQ-based signal transmission and reception operation, the signal transmission apparatus transmits the information part other than the previously transmitted codeword vector, and the additional parity part, i.e., a new parity part, at retransmission. Therefore, the signal reception apparatus can normally restore the information vector using only the retransmitted codeword vector. That is, the signal reception apparatus is self-decodable.
However, because the Type-III HARQ-based signal transmission and reception operation must generate a codeword vector (n,k′) supporting a variable coding rate at retransmission, the signal transmission apparatus should include additional encoders to generate the codeword vector (n,k′) and the signal reception apparatus should also include additional decoders to decode the codeword vector (n,k′). In addition, at retransmission, the signal transmission apparatus transmits the information part and the additional parity part, instead of transmitting the intact codeword vector (n,k′) supporting the variable coding rate, so it must include puncturers for puncturing the remaining parts except for the additional parity vector. As a result, the Type-III HARQ-based signal transmission and reception operation causes an undesirable increase in hardware complexity.